1. Field of the Invention
This invention relates generally to the field of echo cancellers and more particularly to adaptive echo cancellers having dynamically positioned adaptive filter taps.
2. Background of the Invention
Impedance mismatches and other circuit discontinuities in telephone lines often result in the presence of echoes on such lines. Such mismatches generally occur at 2 wire to 4 wire and 4 wire to 2 wire transitions. This is, of course, undesirable in full duplex or even half duplex ping pong type data communications circuits in that such echoes are interpreted by a receiver as noise which can corrupt incoming received signals. Such echo signals may take on many forms with the most common being that of a near end echo (resulting from discontinuities and mismatches at a local telephone office) plus far end echoes (resulting from such discontinuities or mismatches at remote telephone offices). An example of such echoes is shown in FIG. 1 where the near end echo is represented by echo 12 and the far end echo is represented by echo 14.
In the example of FIG. 1, near end echo 12 is shown to be approximately the same magnitude as echo 14. Another possibility is shown in FIG. 2 with echo 16 representing the near end echo and a somewhat smaller echo 18 representing the far end echo.
One of the more common techniques for dealing with such echoes is described in U.S. Pat. No. 4,464,545 to Werner. This echo canceller structure utilizes a near end canceller to cancel the near end echo and a far end canceller to cancel to the far echo. The two "subcancellers" are separated by a bulk delay unit to account for the silent period between the near end and far end echoes. This structure requires knowledge of when in time an echo is likely to occur and how long the echo is likely to last.
U.S. Pat. No. 4,582,963 improves upon this echo canceller arrangement by allowing the bulk delay unit to be variable. In this patent, the variable bulk delay allows for varying distances between the local and remote offices so that the near end canceller and far end canceller can be optimally situated in time to assure the cancellation of both the near end and far end echoes.
Unfortunately, neither of the arrangements shown in the above-referenced patents can account for an echo signal such as that shown in FIG. 3A. In this echo signal, a near end echo 20 is followed by an intermediate echo 22 which is then followed by a far end echo 24. According to published studies by Bell Telephone Laboratories, such intermediate echoes occur in approximately 30% of all data communications situations. The echo cancellers of Werner and U.S. Pat. No. 4,582,963 to Danstrom are unable to cope with such intermediate echoes thus, substantial corruption of data may occur as a result.
In a paper published at the I.C.A.S.S.P. 86 in Tokyo entitled "A Tap Selection Algorithm For Adaptive Filters", Kawamura et al., discuss a tap adaption algorithm entitled "Scrub Taps Waiting In a Queue" or "STWQ" which may allow a digital adaptive filter to ultimately adapt to such an intermediate echo 22 as shown in FIG. 3. Unfortunately, this echo canceller has another drawback encountered by the two previously mentioned types of echo cancellers: namely, that it is undesirable for an echo canceller to operate at times when there is no echo. Running an echo canceller under such circumstances merely creates computational noise as a result of finite word length accuracy in the digital transversal filter. This may actually hinder the reception of data in a marginal line. It is therefore desirable to provide a digital echo canceller structure which overcomes these and other problems associated with conventional echo canceller structures.